The invention relates to a method and an apparatus for detecting the surface topometry of the cornea of the eye with means for a dynamic or static projection of a pattern onto the surface of the cornea and means for detecting the pattern reflected or mirrored by the cornea. The term xe2x80x9clayerxe2x80x9d shall be understood hereinunder within the terms of tomography and is not limited to thin border layers between zones of different refractive indexes.
A wide variety of methods and apparatuses for detecting the surface topography of the cornea are known in which patterns are projected statically (e.g. DE 43 25 494 A1 or U.S. Pat. No. 5,684,562) or dynamically (e.g. DE 43 22 620 A1) onto the cornea and the pattern reflected or mirrored from the cornea is detected. Such methods are usually called video-keratometry or as ring projection according to Placido and have proven their worth in general practice. They allow a point-by-point measurement of the corneal surface (mostly with more than eight thousand measuring points) within a few milliseconds.
The topography of the surface of the cornea is deduced from the position of the projection points and the relative relationship of these points that usually form a ring pattern. The reflected pattern is usually recorded by a CCD array, with the CCD array, e.g. the CCD array of a video camera, usually being disposed coaxially and concentrically at the end of a so-called Placido""s cone. This set-up leads to the consequence, however, that central corneal region which has a diameter of approx. 0.5 mm cannot be detected during the measurement, although it is the central optical zone of the cornea in particular that relevantly determines the refractive power of the eye and typically forms the pass-through point of the visual axis. The so-called Stiles-Crawford effect leads to the consequence that the central corneal zone which is free from any patterns during the projection of Placido""s patterns plays a special role with respect to the peripheral corneal regions in the eye""s projection system.
Moreover, video-keratometry, which occasionally is also designated as video-topography, is unable to supply any information on the back surface of the cornea and the lower sections of the refractive system of the eye, in particular the front side and back side of the lens. The geometry of the boundary surface, the depth of the anterior chamber of the eye, the boundary surface properties and the topography of the lens, the distribution of density and the scattering body arrangements in the lens and the depth of the posterior chamber of the eye (as defined by the distance between lens and retina) are the prerequisites for increasing the precision of refractive measures on the cornea and for implantation-surgical interventions on the lens for the computer-aided detection and analysis of the refractive system of the optical properties of the entired eye in a patient.
However, some of these devices (DE 43 25 494 A1 or U.S. Pat. No. 5,684,562) provide means to alien the projected pattern to the eye or vice versa.
These alignment means may provide certain information about the eye beneath the cornea by processing some light being reflected by inner structures of the eye. Such information is fairly vague especially along the optical axis of the eye.
As an alternative, the U.S. Pat. No. 5,491,524 suggests to use optical coherence tomography (OCT) to provide for topographic results. However, this method is not able to provide information about the geometrical position at which the measurement takes place. Especially, a natural movement of the eye cannot be compensated.
Keeping this in mind, the invention is based on the object of providing a method and an apparatus which allow detecting in a simple and rapid manner both the entire substantial surface topography of the cornea and also at least one optical property of the layers of the eye disposed under the cornea with comparely high accuracy.
This object is achieved by an apparatus of the kind mentioned above in which means are provided for detecting at least one optical property of a layer disposed beneath the cornea of the eye comprising coherence tomographic means. This object is also achieved by a method of the kind mentioned above in which at least one optical property of a layer of the eye disposed beneath the cornea is detected parallel to the detecting process of the surface tomography of the cornea, via coherence tomography.
Such an set-up and such a method enable the required measurements to be performed in one process. This is substantially more pleasant for the person undergoing such a measurement. Moreover, this set-up also ensures that a detection of the optical properties of layers beneath the cornea occurs with a required local calibration because such a local calibration is enabled by the detection of the surface topography of the cornea. It is thus no longer necessary that the person rigidly stares into the focusing light during the measurement of the optical properties of layers disposed beneath the cornea, which is physiological not possible. Rather, smaller deviations can be measured accordingly by the detected surface topography. Thus, the apparatus and the method in accordance with the invention allow determining the optical properties of the entire eye with adequately high local precision for the first time.
For example, the apparatus in accordance with the invention may comprise at least one laser light source, a detector for detecting laser beams generated by the laser light source and means for splitting the beams and deflecting at least a part of the beams into the eye and deflecting onto the detector parts of the laser beams reflected in the eye.
Such an apparatus allows comparing the surface topography and the overall refraction of the eye and thus determining the influence and the data of the optical media disposed deeper in the eye. In addition, scattering image analyses can be obtained from the organ parts of the eye. Particularly the topographically non-detectable central portion can be detected topometrically and topographically with the apparatus.
Accordingly, laser beams can be produced in a method in accordance with the invention by means of at least one laser light source and can be split and deflected in such a way that at least a part of the beams is guided into the eye, with parts of the laser beams reflected by the eye being guided to the detector and being detected by the detector.
Depending on the respective problem to be solved, the method can he performed in such a way that the profile of the wave front of laser beams directed at the eye are compared with the profile of the wave front reflected by the eye. As an alternative or in addition, the method can also be performed in such a way that the running periods of laser beams emitted into the eye are determined.
The measurements will be particularly precise and easy to perform when a Placido Topometer is used for projecting the pattern onto the surface of the cornea. In this process, the laser beam can be guided on their path towards the eye and back through the beam of the Placido Topometer for detecting the optical properties of layers disposed beneath the cornea. For this purpose suitable deflection means such as tilted mirrors or deviating prisms are used.
For determining the optical properties of layers disposed beneath the cornea by it is possible to introduce a known beam profile or a known wave front of a laser source in the zone of the pupillary opening and to direct the same onto the cornea and the lower sections of the eye. By determining the profile form of a wave front reflected by the eye and a comparison with the wave front sent into the eye it is possible to detect the optical properties. A Hartmann-Shack detector is particularly suitable for this purpose. Such an arrangement will yield a particularly precise picture of the optical properties of the layers of the eye which are disposed beneath the cornea.
Alternatively or cumulatively, it is also possible to provide optical coherence tomography (OCT) in order to determine the optical properties of the layers disposed beneath the cornea. Such a coherence tomography has proven its worth and reliably supplies tomographs from the entire eye and also contains information on the layer thickness of the individual portions of the eye which are relevant for the refraction (biometry). In particular, the method and apparatus in accordance with the invention can be used for a substantial improvement of optical coherence tomography because movement artefacts can be respectively corrected by the continuous simultaneous detection of the surface topography of the cornea. Mathematical calculations based on these surface topographies can be used to compensate for corneal movements during data acquisition and can thus also be used to compensate the movements errors for the OCT measurement.
The corneal topography in the central corneal area can also be determined in particular in such a way that in the central Placido-ring-free area of the cornea a short-coherent measurement system, in particular a laser measurement system, is mirrored in and is aimed at the cornea and the lower sections of the eye coaxially to an optical axis extending through the pupil and the retina. In addition, the OCT offers the advantage to measure the morphology and other optical features of the different layers inside the eye. For example, these data can be used to measure and analyze the corneal wound healing process in the stromal tissue after refractive surgery.
As the time needed to capture the OCT information is substantially longer than the topography acquisition time, severalxe2x80x94instead of only onexe2x80x94topographies should be made during the said OCT-acquisition in order to detect and compensate movement artifacts, due to accidental eye movements.
The yet unsolved problem of the alignment of the measuring device with the cornea can be solved as follows: OCT is well as the wavefront sensing device can be correlated to the topographer""s reference point (e.g. the patients line of sight). This is achieved by using the same fixation light for the topography and the wavefront sensing or OCT, enabling to calculate in x/y coordinates the starting point of the data analisis.
OCT also provides morphological information of the relevant optical components of the eye (cornea, lens, vitreous) by acquiring layer-specific information, obtained by appropriate adjustment of the z-axis. In this manner morphological, optical, densitometric data from inside the cornea and the lens can be obtained. As a diagnostic and topometric tool for refractive surgery, only the OCT can provide such data, relevant for diagnosis and therapy. The said morphological information can also be obtained by wavelength selection based on OCT alone or in combination with z-axis variation based data acquisition.
OCT measurements of more than one point (centrally and paracentrally) allow the measurement of the lens in situ. Such measurements can be achieved by splitting the OCT (either statically using prisms, or dynamically using a scanning device) and subsequent assessment of differences ot each beams run-time Beam splitting in one of the described manners allows triangulation measurements of the lens position, an essential morphometric information.
Apart a static projection of the placido ring pattern onto the cornea (tear film), the corneal surface topography as well as the information of the OCT scan can be obtained, using a dynamic projection of one or more light sources. In this manner, the Purcyne images (1=surface cornea, 2=retrocorneal surface, 3=anterior lens surface, 4=posterior lens surface, 5=retina) can be used to measure the said optical relevant surfaces: